This can happen for a few reasons, but the most common would be an "AVS Mismatch". This means that the billing address entered, does not match what your bank has on file for you.
If you check to make sure the billing information entered at checkout matches the address your bank has associated with your credit card, the order process should work smoothly :)
Another option would be to utilize Amazon Pay or PayPal to complete your purchase.
If none of these options are working for you, please feel free to give us a call and place your order by phone. We are available Monday-Friday 10am-4pm PST, please dial 1-866-729-9434.